Heretofore, sound or ultrasonic waves were used to measure the position of a pointing device, where the pointing device transmits acoustic waves which are received by sensors in or above the edges of the writing surface. In other patents, sound waves are traveling across the writing surface, and the pointing device receives them.
However, such prior apparatus presents some implementation problems relating to disturbances in the transmission of the acoustic waves over the air or along the writing surface.
Moreover, there is a need to establish continuous, good physical contact between the pointing device and the writing surface.
Transmitting or receiving means are required on the writing surface, thereby limiting its size.
Another device, the mouse using a trackball, has lower precision because of the ball slipping on the surface, and is more difficult to use than a pen, because it demands wrist movements.